This invention relates to superconductors in general and more particularly to a large current stabilized superconductor with low alternating field losses.
A cryogenically stabilized superconductor in cable form for large currents and alternating field stresses with several superconducting elements which contain twisted conductor filaments of superconductive material embedded in a matrix material of predetermined electric conductivity, the filaments twisted with several stabilizing elements disposed parallel thereto, is described in DE-AS No. 27 36 157. The stabilizing elements are made of thermally and electrically highly conductive material which is electrically normally conducting at the operating temperature of the superconductor. The electric conductivity of the stabilizing elements at the operating temperature is substantially higher than that of the matrix material of the superconducting elements.
The superconductive material of the conductor filaments of the elements of this known large current superconductor may be, in particular, an intermetallic compound of the type A.sub.3 B with an A15 crystal structure such as Nb.sub.3 Sn or V.sub.3 Ga. The superconducting elements of this cable each contain a multiplicity of filaments of such an intermetallic compound embedded in a bronze matrix. Such large current superconductors have good superconduction properties, are distinguished by high critical values and are therefore particularly well suited for the windings of magnets which are used to generate strong magnetic fields. Besides the mentioned superconductive binary compounds, ternary compounds such as niobium-aluminum-germanium Nb.sub.3 Al.sub.0.8 Ge.sub.0.2 can be provided as conductor materials.
To ensure undisturbed continuous operation of a device equipped with superconductors such as a magnet coil or a cable, so-called cryogenic stabilization can be provided. According to this known type of stabilization, electrically and thermally highly conductive material such as copper or aluminum is added to the superconductive material of the conductor. Through good cooling of this normally conducting material, a section in the superconducting material which has become normally conducting can be returned to the superconducting state without interruption of the operation, i.e., the temperature can again drop below the transition temperature of the superconducting material even though the current is maintained.
In the large current superconductor known from DE-AS No. 27 36 157, the stabilization of the superconducting elements is achieved by arranging further special stabilizing elements of normal conducting material parallel to the superconducting elements. The stabilizing elements and the superconducting elements are twisted together to form a flat cable and can be arranged around a carried body in ribbon shape. In this conductor, adjacent superconducting and stabilizing elements are in intimate electrical and thermal contact, obtained from joint hot deformation when the conductor is formed into a flat cable. The known large current superconductor therefore has a low transversal resistance so that it has correspondingly large losses in magnetic fields that change in time.
In proposed applications of superconductive materials in large scale technical installations, requirements have been established for the superconductors to be used, which known conductor configurations do not completely meet in all respects. This relates particularly to the poloidal field coils such as the equilibrium and the transformer coils which are required in a fusion reactor constructed, for instance, according to the Tokamak principle. These coils, which serve for starting up and maintaining a plasma current, for stabilizing the plasma and for the removal of impurities, are advantageously made of superconducting material. In particular, however, these coils must meet the following specific requirements:
(a) Their conductors must carry currents of, for instance, 50 kA, so that the inductance of the coils can be kept low.
(b) Because of the large stored energy of about 10.sup.9 Joule, high cryogenic stability of the conductors must be assured.
(c) The alternating field losses must be kept as low as possible even for high field change rates and amplitudes, for instance, from -7 Tesla to +6 Tesla in one second when the plasma is fired.
The first two requirements call for large conductor dimensions as well as for a large percentage of electrically highly conductive stabilizing material. While both of these requirements can be met with the conductor concept known from DE-AS No. 27 36 157, the third requirement cannot be so met. For, because of the low transversal resistance, the alternating field losses are of a magnitude which can no longer be tolerated. The main cause of these losses are eddy currents which are induced in the stabilizing material, as well as between the superconducting individual wires of the overall conductor.
It is therefore an object of the present invention to improve a superconductor of the type mentioned at the outset in such a manner that it can meet the mentioned specific requirements such as must be imposed on field coils of fusion reactors.